Prior art appliance control systems, such as those for gas-fired water heating appliances, have consisted of separate functional units, including a central control unit, a thermostat, high limit circuitry, safety circuitry, a user interface and a display unit. As a result, it has been difficult to provide a simple and effective self-testing diagnostics system for the entire control system, an informative display unit for displaying detailed operating information, a unified intelligent user interface, and enhanced safety features. Moreover, interfacing and coordinating operation of these separate functional units has been complex, inefficient and costly. Accordingly, there is a need for an integrated appliance control system that is easily adapted for use with a variety of different appliances, is simple to install, customize, operate and maintain, is inexpensive to manufacture, and provides enhanced safety features.
In connection with heating appliances in such fields as water heating, space heating, commercial cooking, and the like, there is often the need for the appliance control system to provide high limit or energy cut-out (ECO) controls, a safety limit string, an igniter current proving circuit, and a flame detection circuit.
ECO controls provide a backup or secondary thermostat function as required by various safety standards or regulations. Typically, ECO controls are of an electromechanical design, such as capillary fluid-filled tubes (which use the principle of fluid expansion to open a microswitch) or bimetallic thermoswitches using dissimilar metals (one of which deforms in the presence of heat) to provide switch contact openings and hence, interrupt power to the gas valve(s) upon reaching a maximum operating temperature.
Both capillary tube thermostats and bimetallic thermoswitch thermostats have significant drawbacks. In this regard, capillary tube thermostats have an inherently unsafe failure mode in that if the copper tube from the sensing bulb becomes fractured (due to fatigue from flexure or vibration), the fluid (upon expansion due to heat) will leak out and have the effect of "looking" like a continuous heat demand to the control.
Bi-metallic thermoswitches suitable for use in commercial hot water heating applications are typically encapsulated into a thermowell assembly. The thermoswitches add a significant cost premium to the control system, and have poor temperature tolerance around the fixed set point temperature (.+-.3 deg. C., typ.). Moreover, applications requiring different high limit temperatures within the same family of appliance often results in the creation of non-standard parts with prohibitive cost and procurement lead times. Another drawback to thermoswitches is their cycle life rating. Generally, thermoswitches are only required to withstand 1000 full-load cycles. Similarly, the load-carrying capability of thermoswitches is limited by their physical size (e.g., 31/2 amps).
Finally, both capillary tube thermostats and bimetallic thermoswitches can be jumpered (i.e., shorted), thus allowing the appliance to exceed the specified safe operating temperature limit.
Safety limit strings cause the immediate shut down of a heating element (e.g., a gas burner or electric heating coil) in response to detection of a malfunction in one of the system components having a corresponding switching device in the safety limit string. Prior art electronic controllers have one or more control board inputs for connecting switching devices (e.g., High Limit/ECO, air pressure switch, gas pressure switch, flow switch, etc.) to the controller (which is typically microprocessor- or microcontroller-based). Switching devices connected to control board inputs can have their status monitored by the controller. However, switching devices connected to the control board inputs are also directly connected into the safety limit string. This dual-purpose connection functionally limits the use of switching devices connected to the controller, since they must also exist within the safety limit string and will interrupt power to a heating element (e.g., a 24 VAC gas valve) in the event of an open switch condition.
If a switching device is meant for use as a means to monitor a condition within the appliance and not meant to provide any limiting control to the heating element, then the switching device must be connected external to the controller (i.e., outside the control board inputs), which in turn limits or eliminates the capability of the controller to monitor the status of a switching device, since the controller can only monitor switching devices physically connected to control board inputs. This prior art control system design can lead to the connection of a large number of non-critical switching devices into the safety limit string, so that the controller can monitor operating conditions within the appliance. As a result, the heating element may be subject to shut-down under conditions which do not necessitate a shut-down.
An igniter current proving circuit is used in a gas-fired appliance which uses a hot surface igniter to ignite a flammable gas (e.g., natural gas). The igniter current proving circuit establishes whether the current provided to the hot surface igniter is sufficient to ignite the flammable gas. If flammable gas is released before the hot surface igniter has become hot enough (from the flow of current) to ignite the gas, there could be a build up of flammable gas that could lead to an explosion or fire. Prior art igniter current proving circuits do not provide means for evaluating the condition of the hot surface igniter for the purpose of maintenance and replacement. Accordingly, there is a need for a igniter current proving circuit having a greater level of intelligence.
A flame detection circuit detects the presence/absence of a flame. If a flame is absent the respective gas valve must be closed to prevent the buildup of gas. Prior art flame detection circuits do not provide means for evaluating the quality of a flame, as well as means for monitoring the degradation of a flame probe located in the flame. Accordingly, there is a need for a flame detection circuit having additional detection features.
The present invention addresses these and other drawbacks of prior art appliance control system designs to provide a control system which has improved intelligence, versatility, convenience, and efficiency.